1. Field of the Invention
The present invention relates to an attitude control method of a spacecraft of an artificial satellite, and more particularly, to a method of improving a maneuverability and a controllability by simultaneously applying a reaction wheel and a thruster among drive units used to maneuver an attitude of the spacecraft of the artificial satellite.
2. Description of the Related Art
Existing driving units mounted on a spacecraft of an artificial satellite include a thruster-based attitude controller and a reaction wheel-based attitude controller. In the related art, the thruster-based attitude controller and the reaction wheel-based attitude controller are independently operated. Generally, the thruster-based attitude controller has a relatively poorer performance in measuring an attitude accuracy of the spacecraft of the artificial satellite, but is used for a safe operation. The reaction wheel-based attitude controller is generally used to improve an attitude control accuracy when photographing for a mission of earth observation.
A technology of using both the existing thruster-based attitude controller and the reaction wheel-based attitude controller may be an angular momentum dumping technology to dump accumulated angular momentum of reaction wheels using the thruster-based attitude controller when the angular momentum of reaction wheels accumulates due to a external disturbance. Specifically, the reaction wheel-based attitude controller is used as a main attitude controller and the thruster-based attitude controller is used as an auxiliary attitude controller.
Here, a thruster is used in the spacecraft for an attitude control and an orbit control, and thus indicates a thrust force generating device to control an attitude or an orbit of the artificial satellite. For example, the thruster used for a gas injection control device may obtain a high temperature and a high pressure gas by causing a chemical reaction or a decomposition reaction using a catalyst in a high pressure gas or liquid, and then may generate a thrust force by quickly spraying the obtained gas via a nozzle. The existing thruster is symmetrically provided with respect to a reference axis of the spacecraft.
A reaction wheel mounted on the spacecraft of the existing artificial satellite denotes a device to generate a torque for the attitude control of the spacecraft, and thus to use a reaction torque occurring by accelerating or decelerating a speed control wheel using an electric motor. A momentum wheel employed in an attitude control system using a bias momentum scheme has the same functions as the reaction wheel. However, the momentum wheel rotates in a single direction, which is different from the reaction wheel.
As shown in FIGS. 1 and 2, reaction wheels are disposed in the spacecraft to be in a pyramid form. Specifically, as shown in FIG. 1, four reaction wheels HRWA1Axis, HRWA2Axis, HRWA3Axis, and HRWA4 Axis are disposed at 90° intervals, and are tilted at an angle of 45° with respect to axes Xsc Axis and Ysc Axis of a reference plane Xsc-Ysc Plane of the spacecraft. As shown in FIG. 2, each of the reaction wheels HRWA1 Axis, HRWA2 Axis, HRWA3 Axis, and HRWA4 Axis is twisted at a torsion angle β with respect to the reference plane Xsc-Ysc Plane of the spacecraft. Each of the reaction wheels has an angle of (90°−β) with an axis Zsc Axis viewing from a camera on the spacecraft.
Here, depending on how to determine the torsion angle of each of the reaction wheels HRWA1 Axis, HRWA2 Axis, HRWA3 Axis, and HRWA4 Axis, an angular momentum or a torque generated by rotating of each of the four reaction wheels HRWA1 Axis, HRWA2 Axis, HRWA3 Axis, and HRWA4 Axis may be differently projected to the axes Xsc Axis, Ysc Axis, and Zsc Axis of the spacecraft.
The four reaction wheels HRWA1 Axis, HRWA2 Axis, HRWA3 Axis, and HRWA4 Axis may be provided in preparation for a failure of at least one reaction wheel by obtaining a marginal degree of freedom (DOF) corresponding to one reaction wheel. For example, in FIG. 1, if one reaction wheel on Hrwa1 Axis fails, it is possible to control three axes (*Xsc Axis, Ysc Axis, and Zsc Axis of the spacecraft) using a limited angular momentum and torques from using the remaining three reaction wheels HRWA2 Axis, HRWA3 Axis, and HRWA4 Axis. In a case where a single reaction wheel is provided on each of three axes, that is, when a total of three reaction wheels are provided on the three axes, respectively, when any one of the three reaction wheels fails, an attitude control may be impossible using the malfunctioning reaction wheel in a corresponding axis. Accordingly, as shown in FIGS. 1 and 2, reaction wheels may be generally disposed in the pyramid form.
In the conventional artificial satellite, even when four reaction wheels are disposed in the pyramid form, a maneuverability of the spacecraft may be significantly deteriorated due to the failure of any one of the four reaction wheels.
FIG. 3 illustrates graphs showing angular momentum envelopes of a spacecraft when all the reaction wheels normally function in the conventional artificial satellite. FIGS. 4A through 4D illustrate graphs showing angular momentum envelopes of the spacecraft when one of the reaction wheels malfunctions.
It can be known from the graphs of FIGS. 3 through 4D that, in a case where any one of the four reaction wheels fails, an angular momentum transferable to reference axes of the spacecraft using the remaining three reaction wheels decreases in comparison to a case where the four reaction wheels normally operate as shown in FIG. 3.